Bay, ocean moisture prevented Sunday's warm-up

A flow of moist air from the Atlantic and Chesapeake Bay kept the Baltimore area from breaking 50 degrees Sunday, when temperatures nearly 20 degrees warmer had been forecast, according to the National Weather Service.

And it's only getting cooler from here on out, for a while at least.

With a large low-pressure system over the Mississippi and Ohio valleys, there was an east-to-west flow of wind over Maryland, explains Greg Schoor, a meteorologist with the weather service's Baltimore/Washington forecast office in Sterling, Va. That brought moist air over the ocean and bay over the region, moving westward all the way up to the Blue Ridge and Appalachian mountains, blocking us from the sun's warming.

"Even though the sun comes up, when you have this moist flow banking up against the mountains, it's like a conveyer belt," Shoor said, with the cloudy, moist air just circulating overhead.

BWI Marshall Airport only reached 49 degrees Sunday, when 66 degrees had been in the forecast as of Friday. Had the sun been able to come out, it could have warmed 10 degrees, Schoor said. Saturday BWI reached a balmy 57 degrees.

On the other side of the mountains, however, in the Shenandoah Valley, areas reached well into the 70s without that cloudy air blocking the sun. Staunton, Va., reached 71 degrees about 3 p.m.

Now, that low-pressure system that was to our west is upon us, bringing more clouds and some rain, though much of the moisture has already been tapped out, Schoor said. Temperatures are expected to peak early in the day Monday, falling into the upper 40s by the evening.

Tuesday begins a stretch of temperatures closer to normal in the 30s and 40s, and more rain is in the forecast.